# Homework Help: I think I understand, but where do you find force?

1. Nov 30, 2015

### Laura0901

1. The problem statement, all variables and given/known data
Find the coefficient of kinetic friction that is used to stop a train engine along its track of the trains velocity is 30 m/s when the brakes are applied, and it takes 30 sec for the 10,000 kg train to stop
2. Relevant equations
Ffric=MN=Mmg
(I think that's the equation to use.

3. The attempt at a solution
Ffric=M(10000kg)(9.8m/s)
Ffric=M(98,000)
What do you use for the force? 30m/s?

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2. Nov 30, 2015

### SammyS

Staff Emeritus
Hello Laura0901. Welcome to PF !

What is the acceleration of the train ?

3. Nov 30, 2015

### Laura0901

It is not said in the problem.

4. Nov 30, 2015

### SammyS

Staff Emeritus
Of course you must figure it out from the given information.

5. Nov 30, 2015

### Laura0901

And how would you do that? Using the kinematic equations?

6. Nov 30, 2015

### Laura0901

Why would we need to find the acceleration? I need the friction force.

7. Nov 30, 2015

### SammyS

Staff Emeritus
And, just what force is it that is slowing the train?

8. Nov 30, 2015

### Laura0901

Either gravity or velocity..I think.

9. Nov 30, 2015

### SammyS

Staff Emeritus
The train slows to a stop. That's an acceleration. That requires a force. (Ask Mr. Newton.)

Gravity is a force, but it acts in a vertical, the train's motion and its acceleration are in the horizontal direction. Gravity is not the force which slows the train.

Velocity is not a force.

10. Nov 30, 2015

### stephen8686

1) Figure out the force in the x-direction that SammyS is talking about, the force that causes the train to stop.
2) Use kinematics to find a
3)Use Fx=max to find the magnitude of the force mentioned in step one
Then you should have enough data to solve Ffric=M(98,000)

11. Nov 30, 2015

### Laura0901

Thanks for trying to help me understand

12. Nov 30, 2015

### Laura0901

There aren't any forces in the x direction. I don't think length is a force. I don't know how to find friction.

13. Nov 30, 2015

### SammyS

Staff Emeritus
Then the train's going to keep going in the x direction.

By the way: What direction is the friction force ?

14. Nov 30, 2015

### Laura0901

The Positive direction

15. Nov 30, 2015

### Laura0901

Is it 2N?

16. Nov 30, 2015

### SammyS

Staff Emeritus
Positive in what sense?
Positive x-direction?
or
Positive as in upward?

17. Nov 30, 2015

### Laura0901

The x direction, it is horizontal

18. Nov 30, 2015

### stephen8686

Okay... I hope you are asking if fk is 2N because M is not in newtons. I don't think fk=2N either though...

19. Nov 30, 2015

### SammyS

Staff Emeritus
Right. It would take very long time to stop.

20. Nov 30, 2015

### Laura0901

Ughh I'm so bad at everything sorry guys.